DC Characteristics of AlGaAs/GaAs/GaN HBTs Formed by Direct Wafer Fusion

We have fabricated AlGaAs/GaAs/GaN heterojunction bipolar transistors (HBTs) formed by direct wafer fusion with different fusion temperatures. By employing a low wafer fusion temperature of 550 $^{\circ}\hbox{C}$, current gains as high as $\sim$9 and output currents as high as $\sim$65 mA (emitter size of $\hbox{100}\times \hbox{120}\ \mu\hbox{m}^{2}$) were obtained. The effective minority carrier lifetime in the base was estimated to have decreased $\sim$20 times due to the fusion process. In comparison, HBTs produced with higher wafer fusion temperatures (600 $^{\circ}\hbox{C}$ and 650 $^{\circ}\hbox{C}$) exhibit lower current gains ($\sim$2–3) and higher base–collector leakage currents.